Mass spectrometer



Nov. 8, 1960 1 M. c. BURK 2,959,676

MASS SPECTROMETER I Filed July 29, 19s? 2 Sheets-Sheet 1 DETECTOR AND STANDARDIZATION CIRCUIT OSCILLATOR INVENTOR. M.C. BURK EMISSION REGULATOR 2 Sheets-Sheet 2 M- C. BURK NECK-00mm Nov. 8, 1960 MASS SPECTROMETER Filed July 29, 1957 INVENTOR. M.C. BU R K A TTORNE VS AAAAA United States Patent 9 MASS SPECTROMETER Marvin C. Burk, Bartlesville, Okla, assignor to Phillips Petroleum Company, a corporation of Delaware Filed July 29, 1957, Ser. No. 674,764

7 Claims. (Cl. 250-41.9)

This invention relates to mass spectrometers and to apparatus for adjusting the sensitivity of mass spectrometers.

In recent years mass spectrometers have been developed from highly specialized academic research instruments for measuring the relative abundance of isotopes into analytical tools of extreme sensitivity and accuracy. At the present time, applications are being found for the use of mass spectrometers in process monitoring and control. Mass spectrometry comprises, in general, ionizing a sample of material under investigation and separating the resulting ions according to their masses to determine the relative abundance of ions of selected masses. The material to be analyzed usually is provided as a gas, which is bombarded by a stream of electrons to produce the desired ions. Although both positive and negative ions may be formed by such electrical bombardment, most mass spectrometers make use of only the positive ions. These positive ions are accelerated out of the region of the electron beam by means of negative potentials. Such potentials impart equal kinetic energies to ions having like charges such that ions of different masses have different velocities after passing through the electrical field and, consequently, have different momenta.

The presently known mass spectrometers can be classified in oneof two general groups: the momentum selection types and the velocity or energy selection types; The momentum selection instruments sort the ions into beams of different masses by the use of magnetic and/or electrical deflecting fields. Ions of selected masses are allowed to impinge upon a collector plate to which is connected an indicating circuit. The velocity or energy selec tion instruments sort the ions according to the velocities or energies imparted to ions of selected masses by electrical accelerating fields.

The present invention can be employed to advantage with either of these types of mass spectrometers.

In accordance with one aspect of the present invention, the collector plate of the mass spectrometer is connected to the control grid of an electrometer tube. The output of the electrometer tube is connected to the input of a DC. amplifier, the output of which is applied to a recorder. A feed-back network is connected between the output of the amplifier and the input of electrometer tube. This permits the gain of the detecting circuit to be adjusted and also stabilizes the operation of the detector circuit. The electrometer tube preferably is mounted within the envelope of the spectrometer tube to minimize losses so that the elfective gain of the detector circuit is increased. A first servo system is provided in accordance with this invention to adjust automatically the zero point of the detector circuit. A second servo system is provided to calibrate the detector circuit when a sample material of predetermined composition is supplied to the mass spectrometer tube.

Accordingly, it is an object of this invention to provide an improved mass spectrometer.

Another object is to provide apparatus for adjusting automatically the sensitivity of the detector circuit associated with a mass spectrometer tube.

A further object is to provide apparatus for calibrating a mass spectrometer.

Other objects, advantages and features of the invention should become apparent from the following detailed description which is taken in conjunction with the accompanying drawing in which:

Figure l is a schematic representation of a mass spectrometer tube having the detector and standardization circuit of this invention associated therewith.

Figure 2 is a schematic circuit drawing of the detector and standardization circuit of this invention.

Referring now to the drawing in detail and to Figure 1 in particular, there is shown a mass spectrometer tube 10 which comprises a gas impermeable envelope, the interior of which is maintained at a reduced pressure by a vacuum pump, not shown, which communicates with the interior of tube 10 through a conduit 11. A sample of gas to be analyzed is directed into tube Ill through a conduit 12. An electron emitting filament 13 is disposed in one end of tube 10 and an ion collector plate 14 is disposed in the second end of the tube. The end terminals of filament 13 are connected to the respective end terminals of the secondary winding 15 of a transformer 16. The end terminals of the primary winding 17 of transformer 16 are connected to an alternating current source 18. The center tap of the secondary winding 15 of transformer 16 is connected to a negative potential terminal 19.

The gas sample supplied through conduit 12 is directed into an ionization chamber 22 in tube 10 which is defined by a pair of spaced grids 23 and 24 that are maintained at ground potential. Electrons emitted from filament 13 are accelerated into chamber 22 by the potential difierence between filament 13 and grids 23 and 24. A grid 26 is positioned between filament 13 and grid 23. Grid 26 is connected to the output of an emission regulator circuit 27, which can be of the type disclosed in the copending application of M. C. Burk, Serial No. 412,790, filed February 26, 1954, now Patent No. 2,792,500, issued May 14, 1957. The input of emission regulator 27 is connected to the center tap of transformer winding 15. This emission regulator is provided for the purpose of applying a potential to grid 26 of magnitude such as to maintain a constant flow of electrons into ionization chamber 22, irrespective of minor fluctuations in the electron emission from filament 13. In this manner the rate at which gas molecules are ionized in chamber 22 by electronbombardment is a function of only the gas pressure in the chamber.

A first collimating electrode 28 is positioned on the second side of ionization chamber 22 and is connected to the contactor of a potentiometer 20. 'One end terminal of potentiometer 20 is connected to a negative potential terminal 21, the second end terminal of potentiometer 20 being grounded. A second collimating electrode 28 is spaced from electrode 28. Electrode 28" is connected to the contactor of a potentiometer 20'. The end terminals of potentiometer 20' are connected to terminal 21 and ground, respectively.

The positive ions produced in ionization chamber 22 are accelerated by the negative potentials applied to electrodes 28 and 28' so as to travel through the tube toward collector plate 14. A first set of three equally spaced grids, 35, 36 and 37 are positioned in tube 10 between grid 28 and collector plate 14. A second set of equally spaced grids 38, 39 and 40 are positioned in spaced relation with the first set of grids; a third set of equally spaced grids 42, 43 and 44 are positioned in spaced relation with Patented Nov. 8, 1960 the fourth set of grids. The spacings s betweengrids and 36, 36 and 37, 38 and 39, 39 and40, 42 and 43-, 43 and 44, 45 and 46, 46 and 47, 49=and50,and'50 and 51 are equal. The spacings r between the centers of grids 37 and 38, 40 and 42, 44 and 45, and 47 and-49 can be represented by the expression:

where n is an integral number.

Grid 35 is connected to the contactor of-a potentiometer 29. One end terminal of potentiometer 29 is connected to a negative potential terminal 30,- and the second end terminal of potentiometer 29 is connected to ground. The contactor of potentiometer 29 can be adjusted to supply selected negative accelerating potentials to grid 35. Grids 36, 39, 43, 46 and 50 are connected to one another and to the first output terminal of a radio frequency oscillator 53, the second output terminal of which is connected to ground. Grid 35 is also connected to a terminal 54. Grids 47 and 49 are connected to one another and to a terminal 55. A direct voltage source, not shown, is applied between terminals 54 and 55 so that terminal 55 is positive with respect to terminal 54. Grids 35 and 51 are connected to one another by a voltage dividing network 56 which comprises five resistors 57, 58, 59, and 61 which are connected in series relationship. Grids 37 and 38 are connected to one another and to the junction between resistors 57 and 58; grids 40 and 42 are connected to one another and to the junction between resistors 58 and 59; grids 44 and 45 are connected to one another and to the junction between resistors 59 and 60; and grids 47 and 49 are connected to one another and to the junction between resistors 60and 61.

A plurality of closely spaced stopping grids 63 are positioned between grid 51 and collector plate 14. Grids 63 are connected to one another and to the contactor of a potentiometer 64. The first end terminal of potentiometer 64 is connected to a positive potential terminal 65, and the second end terminal of potentiometer 64 is connected to ground. A plurality of suppressor grids 66 are positioned between grid 63 and collector plate 14. Grids 66 are connected to a negative potential terminal 67 so as to suppress secondary electrons which may result from ions impinging upon metal parts of the spectrometer tube. A grounded shield 68 is positioned adjacent collector plate 14.

The positive ions formed in chamber 22 are accelerated toward collector plate 14 by the negative potentials applied to grids 28 and 28. During one half cycle of the output signal from oscillator 53, the electrical field between grids 35 and 36 is of such phase that the ions entering this field are accelerated. Ions which enter the field during a particular phase of this half cycle receive maximum energy. During the following half cycle of the signal from oscillator 53, the field between grids 36 and 37 is reversed such that the ions are further accelerated. These ions then drift through the field-free space between grids 37 and 38. The massesof the individual ions determine their times of arrival at grid 38. The ions which arrive at grid 38 at the proper time are again accelerated a maximum amount by the field applied between grids 38 and 39, thereby receiving additional energy. The same accelerating procedure continues as the ions pass through the next ten grids. The positive potential applied to grids 63 is adjusted such that only those ions which receive a predetermined maximum energy are able to pass through grids 63 to impinge upon collector plate 14. The potential applied across the resistance network defined by resistors 57, 58, 59, 60and 61 serves to decelerate the ions so that the velocity of. the Selected ions to which the spectrometer tube is tuned remains substantially constant, whereas the ions of other masses are decelerated in passing through the tube. For a more detailed description of the operation of the mass spectrometer tube thus far described, reference is made to U.S. Patent 2,761,974.

Collector plate 14 is connected to the control grid of an electrometer tube 70, which can be a tube No. 5886 manufactured by Raytheon Manufacturing Company, for example. The several electrodes of this tube are connected to a detector and standardization circuit 71 which is illustrated in Figure 2. The anode of tube 70 is connected through resistors 73 and 74to a positive potential terminal 75. One terminal of the directly heated cathode of tube 70 is connected to ground through a variable resistor 76. The second terminal of the filament is con nected to the junction between resistors 73 and 74 through series connected resistors 77, 78 and 79. The second grid of tube 70 is connected to the junction between resistors 77 and 78. The anode oftube 70 is connected directly to the control grid of a triode 81. The anode of triode 81 is connected through a resistor 82 to terminal 75, and the cathode of triode 81 is connected through a resistor 83 to a negative potential terminal 84. The cathode of triode 81 is connected directly to the cathode of a second triode85. The junction between resistors 78 and 79 is connected directly to the control grid of triode 85. The anode of triode 85 is connected through a resistor 87 to terminal 75. The anode of triode 85 is also connected through a resistor 88 to the control grid of a third triode 89. The control grid of triode 89 is connected through a resistor 90 to terminal 84. The anode of triode 89 is connected directly to ground, and the cathode of triode 89 is connected through a resistor 91 to terminal 84. The cathode of triode 89 is also connected directly to the cathode of a fourth triode 93. The anode of triode 93 is connected through a resistor 94 to terminal 75. A resistor 95 is connected between the anode of triode 81 and the control grid of triode 93. The control grid of triode 93 is connected through a resistor 96 to terminal 84.

The anode of triode 93 is connected through series connected resistors 100 and 101 to the control grid of a triode 102. The junction between resistors 100 and 101 is connected through a resistor 103 to the control grid of a triode 104. The anodes of triodes 102 and 104 are connected directly to terminal 75, and the cathodes of triodes 102 and 104 are connected through a common resistor 105 to terminal 84.

The cathodes of triodes 102 and 104 are connected to a switch a which is adapted to engage terminals 1a, 2a and 3a in respective first, second and third positions. Switch 110a is mechanically connected to switches 11% and 1100. Switch 11% is adapted to engage terminals 1b, 2b and 3b, and switch 1100 is adapted to engage terminals 10, 2c and 3c. Terminal 1:: is connected through a variable resistor and a resistor 116 to ground. The input terminals of a recorder 117 are connected to the respective end terminals of resistor 116. The cathodes of triodes 102 and 104 are also connected to terminal 1b. Switch 11% is connected to ground through a potentiometer 118. The contactor of potentiometer 118 is connected to the control grid of electrometer tube 79 through a resistor 120.

Electrometer tube 70 and triodes 81, 85, 89, 93, 102 and 104 form a high gain D.C. amplifier which provides an output signal respective of the ions impinging upon collector plate 14. When switches 110a and 11012 engage respective terminals 1a and 1b in a first position, the output signal from this amplifier is applied to recorder 117. Variable resistor 115 permits the amplitude of the recorded signal to be adjusted. The gain of the amplifier circuit can be adjusted by means of potentiometer 118. It can be seen that this potentiometer-forms apartofa feed-back network between thepoutputof the amplifien-j.

and the input of the electrometer tube. The gain of the amplifier circuit is thus determined in part by the setting of potentiometer 118.

Terminals 3a and 2a are connected to one another and to ground through the winding of a polarized relay 121. Relay 121 is provided with a switch arm 122 which engages a terminal 123 when current of a first polarity is applied to the relay winding and a terminal 124 when current of a second polarity is applied to the relay winding. Terminals 123 and 124 are connected to respective switches 125 and 126 which are actuated by a relay 127. Switches 125 and 126 engage respective terminals 128 and 129 when relay 127 is de-energized and terminals 130 and 131 when the relay is energized. The first terminal of the winding of relay 127 is connected to the first terminal of alternating current source 133. The second terminal of the coil of relay 127 is connected to terminal 20. The second terminal of current source 133 is connected to switch 1100.

The standardization circuit is provided with first and second shaded pole induction motors 134 and 135. The first terminal of the line winding 136 of motor 134 is connected to the first terminal of current source 133. The second terminal of motor winding 136 is connected to terminal 3c. The first terminal of the line winding 137 of motor 135 is connected to the first terminal of current source 133. The second terminal of motor winding 137 is connected to terminal 20. The end terminals of the second winding 138 of motor 134 are connected to respective terminals 128 and 129. The end terminals of the second winding 139 of motor 135 are connected to respective terminals 130 and 131. The center taps of motor windings 138 and 139 are connected to switch 122. Terminal 2]] is connected to the contactor of a potentiometer 140. The first end terminal of potentiometer 140 is connected to a negative potential terminal 141, and the second end terminal of potentiometer 140 is connected to ground.

The zero adjustment of the detector circuit is performed automatically when switches 110a, 1101) and 110s are moved into engagement with respective terminals 3a, 3b and 3c in the third position. Potentiometer 64 of Figure 1 is adjusted at this time so as to prevent ions from impinging upon collector plate 14. The output signal from electrometer tube 70 should thus be zero. If there is an output signal from triodes 102 and 104, relay 121 is energized so that switch 122 engages either terminal 123 or 124, depending upon the polarity of the output signal from the amplifier circuit. At this time, current is supplied to winding 136 of motor 134. Relay 127 is de-energized so that motor winding 138 is connected in circuit with relay 121. If relay 121 is energized, motor 134 rotates in one direction or the other to adjust variable resistor 76 which determines the cathode bias on electrometer tube 70. The direction of rota-tion of motor 134 is such as to adjust resistor 76 until the output signal from the amplifier circuit is zero. This de-energizes relay 121 to terminal rotation of motor 134.

The mass spectrometer can be calibrated periodically by supplying a gas of predetermined composition to inlet conduit 12 of Figure 1. The standardization is accomplished by moving switches 110a, 110b and 110c into engagement with respective terminals 2a, 2b and 2c in the second position. This applies a negative potential of predetermined amplitude from potentiometers 140 and 118 to the control grid of triode 70. Motor winding 137 is energized at this time because switch 1100 is in engagement with terminal 20. If the potential at the cathodes of triodes 102 and 104 dilfers from ground potential, relay 121 is energized so that switch 122 engages one of the terminals 123 and 124, depending upon the polarity of the signal applied to the winding of relay 121. Relay 127 is energized at this time so that motor winding 139 is connected in circuit with relay 121. Motor 135 thus adjusts the setting of feed-back potentiometer 118 until 7 determined composition and is capable of obtaining the zero point automatically. It should be evident that switches a, 110b and 1100 can be actuated in sequence if desired in an automatic manner by means of a conventional timer. This timer can be synchronized with the sample inlet system so that the instrument is calibrated periodically. It has been found that the positioning of electrometer tube 70 and grid resistor within tube 10 increases the output and stability of the instrument. Resistor 120 is extremely large, 10 ohms, for example.

While the invention has been described in conjunction with a present preferred embodiment, it should be evident that it is not limited thereto.

What is claimed is:

1. A mass spectrometer comprising an ionization chamber; a collector plate spaced from said ionization chamber; means to direct ions of a predetermined massfrom'said ionization chamber to said collector plate; amplifying means having the input thereof connected to said collector plate to provide an output signal representative of ions impinging upon said collector plate; a recorder; a feedback network; a reference potential; selector switching means to connect said recorder to the output of said amplifying means and to connect said feedback network between the output and input of said amplifying means in a first position, to apply said reference potential through said feedback network to the input of said amplifying means in a second position, and to disconnect said feedback network from the output of said amplifying means in a third position; means responsive to the output signal of said amplifying means in said second position to adjust said feedback network; and means to adjust the gain of said amplifying means in said third position responsive to the output signal of said amplifying means.

2. The mass spectrometer of claim 1 wherein said means to adjust said feedback network comprises a shaded pole induction motor, means to apply a source of alternating potential across the line winding of said motor, and means to connect the center of the second winding of said motor selectively to one end thereof, the end of said second winding to which the center is connected depending upon the amplitude of the output signal of said amplifying means.

3. The mass spectrometer of claim 1 wherein said means to adjust the gain of said amplifying means comprises a shaded pole induction motor, means to apply a source of alternating potential across the line winding of said motor, and means to connect the center of the second winding of said motor selectively to one end thereof, the end of said second winding to which the center is connected depending upon the amplitude of the output signal of said amplifying means.

4. A mass spectrometer comprising an ionization chamber; a collector plate spaced from said ionization chamber; means to direct ions of a predetermined mass from said ionization chamber to said collector plate; amplifying means having the input thereof connected to said collector plate to provide an output signal representative of ions impinging upon said collector plate; a recorder; a feedback network; a reference potential; selector switching means to connect said recorder to the output of said amplifying means and to connect said feedback network between the output and input of said amplifying means in a first position, and to apply said reference potential through said feedback network to the input of said amplifymg means in a second position; and means responsive 5. A mass spectrometer comprising an evacuated. vessel enclosing an ionization chamber, a collector. plate spaced from said ionization chamber, and an electrometer tube. having the input thereof connected to said collector plate;

means to direct ions of a predetermined mass. from said ionization chamber to said collector plate; amplifying means having said electrometer tube as the first-stage thereof to provide an output signal representative of :ions. impinging upon said collectorplate; a recorder; a feedback network; a reference potential; selector switching means to connect said recorder to the output of said amplifying w means and to connect said feedback network between.

the output and input of said amplifying means in afirst. position, to apply said reference .potentialthrough saidfeedback network to the input of said amplifying means in a second position, and to disconnect said feedback network from the output of said amplifying means in a third position; means responsive to the output signal of said amplifying means in said second position and to adjust said feedback network; and means to adjust the gainof said amplifying means in said third position-responsive to the output signal of said amplifying means.

6. A mass spectrometer comprising an evacuated vessel enclosing (1) an ionization chamber, (2) a collector plate spaced from said ionization chamber, and (3). an electrometer tube having the input thereof connectedto said collector plate; means to direct ions of a predetermined mass from said ionization chamber to said collector plate;

amplifying means having said electrometer tube as the first stage thereof to provide an output signal representative of ions impinging upon said collector plate; arecorder; a feedback network; a reference potential; selector switching means to connect said recorder, to the output of said amplifying means and to connect said feedback net-.

work between the output and input of, said amplifying means in a first position, and to apply saidreference potential through said feedback network to the input of said amplifying means in a second position; and means re sponsive to the output signal of said amplifying means in said second position to adjust said feedback network.

7. The mass spectrometer of claim 6 wherein said means to adjust said feedback network comprises a shaded pole induction motor, means to apply a source of alternating potential across the line winding of said motor, and means to connect the center of the second winding of said motor selectively to one end thereof, the end of said second winding to which the center is connected depending upon the amplitude of the output signal of said amplifying means.

References Cited in the file of this patent UNITED STATES PATENTS 2,551,637 Robinson May 8, 1951 2,734,949 Berry Feb. 14, 1956 2,756,347 White July 24, 1956 2,846,586 Jernakoif Aug. 5, 1958 

